Gaseous streams often carry particulate material derived from the environment or from some physical operation. Commonly oil mists can be formed from synthetic petroleum or natural oils. In many instances, removal of some or all of the particulate material from a volume of gas or from gas flow stream, (i.e.) the atmosphere in a work locus, is important for aesthetic or operational purposes. In the case that a lubricant mist is entrained in the air, such mist removal can be essential for worker safety or operating efficiency. For example, in an environment wherein a thermal process involves liquid processing or a mechanical or machine operation uses a stream or film of liquid lubricant, the energy of these operations can, as a byproduct, produce an airborne mist of the lubricant liquid. Such airborne mists can be an irritation or harmful to human operators or any personnel coming into contact with important amounts of the mist. Such mists can also impact efficiency of or harm equipment.
In the past, filter structures have been used with fan driven air streams in housings and ductwork to attempt to remove such mists from the environment. We have found, however, that many installed mist collection systems are not as effective as needed in many environments. Many and return a substantial proportion of the mist to the atmosphere. Other more efficient systems, that trap and remove a substantial proportion of the mist, can become quickly saturated and rendered ineffective after a brief period of effective mist removal. This is particularly true in the case of an effective installation system utilizing a HEPA filter in a final stage. If the filtering stages preceding the final HEPA filter are not sufficiently efficient, the final HEPA filter stage can become rapidly saturated, requiring uneconomical, rapid replacement.
Accordingly, a substantial need exists for an efficient filter medium and a filter structure containing the media that can provide improved properties for filtering gaseous streams carrying a mist. The improved systems will remove substantially all mist and have an extended life. The pupil mist will have a particle size about 0.1 to 3 microns, often 0.2 to 0.8 and commonly 0.3 to 0.5 microns. Such a media and the filter structure containing the media can be used in a staged mist collection system having an initial barrier and a pre-filter prior to the filter of the invention. A high efficiency (HEPA) filter can follow the filter of the invention. In such a system, an initial stage can remove large liquid or solid particulate, a subsequent stage can collect substantially all of the mist components (at least 75%, preferably 88% or more) and a final stage utilizing a HEPA filter can polish the gaseous stream removing a substantially all traces of the mist components. Such systems should maintain an effective life, useful lifetime or effectiveness for more than two weeks, preferably more than a month.